When the Diamondbacks traded third baseman Martin Prado to the New York Yankees at the July 31 trade deadline, Jake Lamb thought his turn had come. The 23-year-old, then with Double-A Mobile, was sure he'd be getting the call to the majors.

"I'd be lying if I said I wasn't thinking about it," Lamb said. "I was in the trainer's room talking with one of our trainers, and one of the guys came in and showed me the Twitter feed. I started shaking a little bit."

His instincts were right, but his timing was off. After a brief promotion to Triple-A, the Diamondbacks on Thursday promoted Lamb to the big leagues, designating Andy Marte for assignment to make room on the active and 40-man rosters.

Lamb is the team's best positional prospect after putting together a .327/.407/.566 line between Double-A and Triple-A, and he'll get the rest of the season to prove he belongs in the majors. After the trade of Prado and the retirement of Eric Chavez, Lamb is the only true third baseman on the roster.

Jordan Pacheco and Cliff Pennington may see some time there, but only to avoid placing too much of a burden on the rookie.

"It gives him a chance at least in a situation where there's not a lot of pressure based on where our team's at right now," General Manager Kevin Towers said. "Next year going into spring training he'll probably get an opportunity to be the everyday guy. This experience will be good for him."

Lamb was a sixth-round pick by the Diamondbacks in the 2012 draft out of the University of Washington. He's hit at every stop he's made in the minors, posting a career .959 OPS, although there were questions about his durability after an injury to his hand and a nagging hamstring limited him to 64 games with High-A Visalia in 2013.

Lamb is healthy now, and credits his personal trainer he hired in the offseason for getting him in better shape. He played in the Arizona Fall League after the 2013 season and earned his first invite to big-league spring training in 2014, where he took advantage of the tutelage offered by Prado and Chavez.

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The slimmer build and another year of seasoning has helped turn him into a stronger defender. Lamb had 12 errors in 55 games at third last year; in 2014 he had 11 in 102. He was tested early Thursday, rushing up for a dribbler on the first batter of the game and snagging a line drive on the next one.

He also added an RBI single in his second at-bat.

Lamb's call-up signals an increasing emphasis on seeing what the organization's young players can do before next season. When asked about the possibility of seeing Lamb in the majors in the aftermath of the Prado trade, Towers was lukewarm on the idea. He cautioned that the team did not yet need to place him on the 40-man to protect him from the Rule 5 draft and may need to use available slots to protect those who were draft eligible, suggesting that Lamb's chance may have to come in 2015 spring training.

A week of Lamb in Triple-A — where he hit .500 with a 1.460 OPS in five games — may have changed that time line.

"Based on where we're at and the way things were going, it was like, 'Why not now?' " Towers said. "We wanted to make sure mentally he could handle it. Everybody felt like now was the time, not pushing the issue or the envelope where we didn't think he was ready."

The youth movement didn't exactly start with Lamb, and it won't end with him either. The Diamondbacks have already heavily relied on young players, with a National League-high 379 combined games played by rookies. Lamb is the fourth Diamondbacks rookie to start the season in Double-A, joining right-handers Chase Anderson and Matt Stites and outfielder David Peralta.

But Lamb is one of the first call-ups – along with Stites – aimed purely at seeing what a prospect can do in the majors. Arizona could have trudged through the rest of the season with a combination of Pennington, Pacheco and Marte at third and waited on starting Lamb's service clock, but felt it was more important to gauge his progress with some big-league action.

Arizona may take a look at more of their prospects – especially pitchers – even before rosters expand in September.

"You may see other guys come up under the same circumstances and try to figure out what their abilities are and how they can handle this environment," manager Kirk Gibson said.

Notable: Anderson has allowed two earned runs or fewer in 10 of his 13 starts, and is 1-0 with a 1.80 ERA in his past four. He's faced the Rockies once, giving up one run on four hits in six innings on June 3. He's still being hurt by the long ball — only four of his 13 starts have been homerless. …. Matzek is a former first-rounder from the 2009 draft who was called up in June to replace the injured Eddie Butler in the rotation. He's never had a particularly strong track record in the minors, with a career 4.34 ERA over five minor-league seasons, but has cut down his walks in 2014. That didn't help him in his last outing, though, when he gave up eight runs on 10 hits in four innings against the Detroit Tigers.